Contact assembly

ABSTRACT

A contact assembly is provided for connecting pairs of terminals of two circuits, which allows terminals to be positioned at high densities with minimum crosstalk, selected impedances, and minimum inductances for fast pulse rise times. The apparatus includes a dielectric frame (30, FIG. 2) having multiple miniature cavities (46), and an extendable contact assembly (26) lying in each cavity and having a pair of probes (32, 34) projecting from opposite faces of the frame. The two probes of each contact assembly are slideably engaged and are biased apart by a spring (70), the spring being constructed of dielectric material to avoid inductances and to allow for a high characteristic impedance. The frame can include a body (104, FIG. 6) having multiple plated through holes or cavities (106), and end wall structures (114) having dielectric bushings (122) shorter than the body holes and fitting into the body cavities.

This is a continuation of application Ser. No. 535,689 filed Jun. 11,1990 now abandoned.

BACKGROUND OF THE INVENTION

Modern high speed, high density circuitry often requires contacts withvery close center-to-center spacings of less than 0.050 inch. Such closespacing can be achieved by spring probe compression contacts, whereineach contact has a spring biased probe whose tip can press against aconductive pad on a circuit board. U.S. Pat. No. 4,200,351 shows acompression contact wherein a spring-biased plunger projects from oneface of a dielectric support to contact a conductive pad on a circuitboard.

While compression contacts allow for high densities of contacts, theycan create problems. The close spacing of the contacts can lead tocrosstalk. Also, if each contact lies close to a grounded shield, thecharacteristic impedance of the contact may be too low. It may be notedthat coaxial cable configurations generally employ three impedancelevels: 50, 70, and 93 ohms. Any metal spring around the contactincreases its effective diameter and therefore decreases thecharacteristic impedance. Furthermore, any metal spring can addappreciable inductance to the contact which is detrimental to thetransmission of high speed data signals. Many present applicationsrequire a pulse rise time of less than 32 picoseconds, which cannot beachieved if there is appreciable inductance. A contact arrangement whichallowed close spacing of contacts, while minimizing crosstalk,minimizing inductance, and allowing control of impedance especially toallow for relatively high impedance, would be of considerable value.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a contactapparatus is provided to facilitate high contact densities, whileminimizing crosstalk and inductances, and which can facilitate controlof impedance. The apparatus can include a frame of dielectric materialhaving a plurality of cavities that each hold an extendable contactassembly. Each contact assembly includes at least one probe projectingthrough a hole in the frame to project from a face of the frame, theprobe being deflectable when a contact pad is pressed thereagainst. Aspring device that biases the probe is preferably formed of dielectricmaterial to minimize inductance and allow for a small diameter contactassembly. The contact assembly can include two probes with tips thatproject from opposite faces of the frame, with the two probes beingpermanently slideably engaged.

The contact apparatus can include a body with plated-through holes andend wall structure with bushing portions that fit into the holes. Thelength and dielectric constant of the bushing portions can control thecharacteristic impedance of each coaxial conductor formed by a contactassembly in the hole and the grounded plating that lines the hole.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a circuit assembly whichincludes a compression contact apparatus of the present invention and apair of circuit boards that can be interconnected through the apparatus.

FIG. 2 is a sectional view of the compression contact apparatus of FIG.1, showing two contact assemblies, with one in an uncompressed state andthe other in a fully compressed state.

FIG. 3 is a plan view of the compression contact apparatus of FIG. 1,with a portion as seen on line 3--3 of FIG. 2 and another portion asseen with the upper wall of FIG. 2 removed.

FIG. 4 is a partial sectional view of another contact apparatus withanother form of spring device.

FIG. 5 is a partial section view of contact apparatus with still anotherform of spring device.

FIG. 6 is a sectional view of a contact assembly constructed inaccordance with another embodiment of the invention.

FIG. 7 is a partial perspective exploded view of the assembly of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a circuit assembly 10 which includes a compressioncontact apparatus 12 of the present invention which can be used tointerconnect a pair of electrical circuits such as those on a pair ofcircuit boards 14, 16. The first circuit board 14 has terminals orcontact pads 20 which are to be connected to corresponding pads 22 onthe other circuit board. The contact apparatus 12 has numerous contactassemblies 26 that can interconnect pairs of corresponding pads 20, 22.Each contact assembly 26 lies in a dielectric frame 30 and has a pair ofprobes 32, 34 projecting from opposite faces 36, 38 of the frame.

As shown in FIG. 2, the frame 30 has first and second ends walls 40, 42and has side walls 44 extending between the end walls. The frame formsmultiple cavities 46 between the walls, with each contact assembly 26lying largely in one of the cavities. The end walls 40, 42 each hasmultiple holes 50, 52, and the probes 32, 34 project from the cavitythrough corresponding holes in the end walls, and initially beyond theopposite faces 36, 38 of the frame.

The second probe 34 of each contact assembly includes a lower or outerportion 54 and an upwardly extending pin portion 56. It should be notedthat the terms "lower", "upwardly", etc. refer to parts as illustrated,but not necessarily to their orientation in actual use. The first probe32 includes an upper or outer portion 60 which has a bore 62 thatslideably receives the pin portion 56 of the other probe. The bottom orinner portion 64 of the first probe forms fingers that resilientlyengage the pin portion 56 to maintain electrical contact between the twoprobes 32, 34 as they telescope, or slide together and apart.

Each probe has a flange 66, 68 fixed to the rest of the correspondingprobe. A spring device 70, shown here as a coil spring, extends betweenthe flanges of the two probes. The spring is preloaded in compression,so it constantly urges the tips 72, 74 of the probes away from eachother and outward from a corresponding face of the frame.

When the two circuit boards 14, 16 are pressed against the oppositefaces 36, 38 of the frame, they deflect the probe tips inwardly untilthey lie substantially flush with the faces of the frame, with the tipsof the probes contacting the conductive pads 20, 22 on the circuitboards. In this way, the pads 20, 22 of the circuit boards areinterconnected. During inward movement of the probes, the spring 70 iscompressed and the pin portion 56 of the lower probe slides more deeplyinto the bore 62 of the upper probe.

The use of compression contacts allows the contact assemblies to lieclose together. This can lead to crosstalk between adjacent contacts. Tominimize such crosstalk, applicant forms the side walls 44 to provide aseparate side enclosure 76 around each contact assembly, and plates theoutside of each side enclosure with a plating 80 of conductive material.Inasmuch as the plating around all contact assemblies are at the samepotential such as ground, they are interconnected. The plating isconnected to a ground terminal of one or both of the circuit boardsthrough a contact (not shown).

Where the contact assemblies carry high frequencies or pulses with shortrise times (current rise times are often less than 32 picoseconds) it isimportant that each contact assembly have minimum inductance, inasmuchas inductance tends to filter out higher frequencies and increase therise time of pulses. A potential source of high inductance is the coilspring 70. Applicant avoids the introduction of inductance from the coilspring 70 by constructing the spring of dielectric material such as ofNylon which is a nonelastomeric plastic. By avoiding the presence of along and large diameter electrically conductive element in the contactassembly, applicant also produces a contact assembly of small equivalentoutside diameter, the equivalent outside diameter being somewherebetween the outside diameter of the pin portion 56 and of the outerportions 54, 60 of the probes.

It is often desirable to establish a controlled characteristic impedancefor each contact assembly. Coaxial impedance cables generally have animpedance of 50, 70, or 93 ohms, and it is often desirable to match thecharacteristic impedance of the contact assemblies to that of coaxialconductors on the circuit boards. The characteristic impedance of acontact assembly may be lower than desired. The impedance can beincreased by increasing the distance between the center conductor,formed by the contact assembly 26, the outer conductor formed by theplating layer 80 surrounding it. By avoiding a spring of electricallyconductive material, applicant produces a contact assembly of smallequivalent outside diameter, to enable a higher characteristic impedanceto be produced. It is possible to construct the flanges 66, 68 that areattached to the probes, of dielectric material, to further decrease theequivalent outside diameter of each contact assembly.

FIG. 4 illustrates another compression contact apparatus 90 that issimilar to that of FIGS. 1-3, except that it uses a spring device 92formed by a bellows of dielectric material such as Nylon. FIG. 5 is aview of still another contact apparatus 92, which is similar to that ofFIGS. 1-3, except that its spring device 96 is formed by a tube ofelastomeric material such as rubber.

FIG. 6 illustrates another contact apparatus 100 which includes a frame102 that can hold contact assemblies such as a pressure contact assembly26A that is similar to that of FIGS. 1-5. The frame 102 includes a body104 with multiple through holes or cavities 106. Two end wall structures110, 114 lie against the opposite faces 116, 118 of the body. Each endwall structure includes bushing portions 120, 122 that fit into thecavities 106. For the pressure contact assembly 26A, the bushingportions form holes 124, 126 that receive end portions of a dielectricspring 130 to control the position of the spring.

Each cavity 106 has an internal surface portion 132 formed of metal,which can be achieved by molding the body 104 of plastic and plating it.The surface portions 132 of some or all of the cavities areinterconnected, as by extending the plating as at 134 to interconnectthe plating portions at the insides of different cavities. The platingis at a controlled potential, such as ground. This forms a coaxialconductor at each contact assembly, with the metal surface portion orplating 132 forming the grounded outer conductor and the contactassembly such as 26A forming the inner conductor.

It is often important to construct coaxial cables and connectors so theyhave a predetermined impedance that matches the impedance of other partsof a circuit connected to it. The use of bushing portions 120, 122 thatlie between the contact assembly such as 26A and the grounded plating132 enables close control of the characteristic impedance of eachcoaxial connector formed by a contact assembly 26A and a grounded layer132. This is because the characteristic impedance of a coaxial conductoris dependent on the material between its conductors. Specifically, thecharacteristic impedance is proportional to the inverse of the squareroot of the dielectric constant of material between the inner and outercoaxial conductors. Where a high characteristic impedance is desired,the bushing portions 120, 122 can be of material of low dielectricconstant, and can be short so that almost the entire volume between theinner and outer conductors is filled with air. Where a lowcharacteristic impedance is desired, the bushing portions 120, 122 canbe long so their combined length is almost equal to the length of theholes 106, and the bushing portions can be formed of material of highdielectric constant. In any case, the bushing portions are formed ofdielectric material.

The strength of the contact assembly frame 102 is primarily in the body104, so the end wall structures 110, 114 and their bushing portions canbe constructed of low strength material. It may be noted that the body104 can be constructed entirely of metal, although it is generallyeasier to form it of molded plastic and to plate the plastic.

Although the pressure contact assemblies 26A can be used in the frame102, the frame can also be used to hold other contact assemblies such asone illustrated at 140 which is designed to receive pin contact such asillustrated at 142.

Thus, the invention provides a contact apparatus which is of simpleconstruction and which has minimal inductance. The apparatus can includecompression contact assemblies lying in cavities of a frame, with eachcontact assembly having at least one probe tip projecting from a face ofthe frame but being resiliently depressible inwardly. A spring thatbiases the probe outwardly, is formed of dielectric material. Eachcontact assembly can include two probes projecting from holes inopposite end walls of the frame and with the probes in permanent slidingcontact with each other. A variety of dielectric spring devices can beused to outwardly bias the probes, including a coil spring, a bellows, atube of elastomeric material, or a collapsing hourglass shape. The framecan include a body with plated cavities, and end wall structures havingbushings that project into the cavities and control the characteristicimpedance.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art, and consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

I claim:
 1. Compression contact apparatus comprising:a frame havingfirst and second end walls, a plurality of cavities between said endwalls, and a plurality of holes in each end wall, each hole extendinginto one of said cavities; an extendable contact assembly lying in eachof a plurality of said cavities, each contact assembly including a pairof probes projecting through holes at opposite ends of a correspondingone of said cavities and electrically connected together, and a springdevice in said cavity which urges said pair of probes apart; each ofsaid spring devices comprising a coil spring constructed of anonelastomeric plastic dielectric material to avoid the passage ofcurrent through said coil spring.
 2. Compression contact apparatus forconnecting pairs of conductive pads on two circuit boards or the like,comprising:a frame of dielectric material having parallel first andsecond opposite faces; a plurality of contact assemblies, each havingfirst and second probes with tips projecting from said opposite facesbut being moveable into a corresponding frame face by a conductive padpressed towards the corresponding frame face and being moveable out ofthe corresponding frame face, said first probe having a bore in an endopposite its tip, and said second probe having a pin portion slideablyreceived in the bore of the first probe; each contact assembly alsoincluding a spring device biasing said first and second probes to urgetheir tips out of corresponding frame face.
 3. The apparatus asdescribed in claim 2 wherein:the spring device of each contact assemblyis constructed of dielectric material.
 4. Contact apparatus comprising:aframe which includes a body having a face and a plurality of cavitiesextending into said face, and at least one wall structure lying oversaid face and having a plurality of apertures for passing contacts, saidwall structure including a plurality of dielectric bushing portionsprojecting into said cavities; a plurality of contact assemblies, eachlying in one of said holes; said cavities having walls of electricallyconductive material, with the conductive material of said plurality ofcavities connected together, but with each contact assembly beingelectrically isolated from the conductive material at the walls of itscavity.
 5. Compression contact apparatus comprising:a frame ofdielectric material having first and second end walls, a plurality ofcavities between said end walls, and a plurality of holes in each endwall, each hole extending into one of said cavities; an extendablecontact assembly lying in each of a plurality of said cavities, eachcontact assembly including a pair of probes projecting through holes atopposite ends of a corresponding one of said cavities, and a springdevice constructed of dielectric material in said cavity which urgessaid pair of probes apart; said first probe has walls forming a borewith an opening in one end thereof which is closet to second probe, andsaid second probe having a pin slideably received in said first probebore; said first and second probes each have a flange lying in saidcavity, and said spring device lies about said pin and has opposite endsbearing against said flanges.
 6. Compression contact apparatuscomprising:a frame of dielectric material having first and second endwalls, a plurality of cavities between said end walls, and a pluralityof holes in each end wall, each hole extending into one of saidcavities; an extendable contact assembly lying in each of a plurality ofsaid cavities, each contact assembly including a pair of probesprojecting through holes at opposite ends of a corresponding one of saidcavities, and a spring device constructed of dielectric material in saidcavity which urges said pair of probes apart; said frame includes aplurality of side enclosures projecting from said second end wall, eachside enclosure surrounding one of said contact assemblies, and said sideenclosures being spaced apart; and including conductive materialcovering the outside of each side enclosure.
 7. Contact apparatuscomprising:a frame which includes a body having a face and a pluralityof cavities extending into said face, and at least one wall structurelying over said face and having a plurality of apertures for passingcontacts, said wall structure including a plurality of dielectricbushing portions projecting into said cavities; a plurality of contactassemblies, each lying in one of said holes; said cavities having wallsof electrically conductive material, with the conductive material ofsaid plurality of cavities connected together; said body has apredetermined thickness and said cavities extend through the entirethickness of said body, said body having first and second faces; saidframe includes a second wall structure lying over said second body faceand having apertures and having bushing portions projecting into saidcavities.